Electronic hearing protector with quadrant sound localization

ABSTRACT

An electronic hearing protector includes an ear cup assembly comprising a front exterior microphone that provides a front microphone signal and a rear exterior microphone that provides a rear microphone signal. A processor receives and digitizes the front microphone signal and the rear microphone signal, and provides a front channel signal and a rear channel signal indicative thereof respectively. A filter receives the rear channel signal and has a cut-off frequency value that provides a high-frequency roll-off and a notch at a notch filter frequency value that is less than the cut-off frequency value, where the filter provides a filtered signal. A first signal indicative of the front channel signal and a second signal indicative of the filtered signal are summed, and a signal indicative of the summed signal is provided to a speaker within the ear cup that provides an audio signal within the first ear cup indicative of the summed signal.

FIELD OF TECHNOLOGY

The present disclosure relates to the field of electronic hearingprotection, and in particular to electronic hearing protectors thatinclude quadrant sound localization.

RELATED ART

Humans (and other animals) use two ears to localize sounds from left toright and from front to back. A sound source at a given angle willarrive at the closer ear earlier and typically at a stronger level, andwill arrive at the farther ear later and typically at a weaker level.These binaural cues are known as the interaural time difference (ITD)and the interaural intensity difference (IID). When a sound source isdirectly in front of the listener, the levels and arrival times areequal.

This simplification, referred to as “duplex theory”, allows forlocalization of sounds in the horizontal plane in front of the listener,but there remains some ambiguity if sounds from the rear are also ofinterest. For example, a sound originating from a point directly aheadof the listener would have roughly the same ITD and IID as a soundoriginating from a point directly behind the listener at the samedistance. In fact, shifting the sound source 30 degrees to the left inboth the front and rear would result in a similar ambiguity. In otherwords, duplex theory is insufficient for front-to-back localization ofsound, and also for localization in elevation. For every off-axis angle,there exists a set of points resembling a cone with an axis through bothears, known as the “cone of confusion”, where a sound coming fromanywhere on that cone will not be resolvable within that cone.

Additional factors beyond those identified by duplex theory allow forbetter localization of sound. Spectral shaping of sounds due toreflection and refraction around and along the head, torso, and outerear (pinna) may be used to infer information about the location of thesource. For example, a sound originating from behind the listener musttravel through and around the tissue of the outer ear, or pinna, whichwill act as an acoustic filter. This filter will shape the spectrum ofthe original sound, which can be used by the brain as a cue to thelocation of the sound source. The characteristics of the variousspectral cues encoded in a sound by the head, torso, and ears of alistener are known as head-related transfer functions (HRTF), and varyfrom person to person.

Modeling of spatial cues via HRTFs has been used to convert surroundsound content to stereo, so that spatial information from a game or amovie is encoded within two channels. However, since HRTFs vary fromperson to person, a given HRTF measured and encoded for one listener maynot always recreate a the correct spatial sensation for a differentlistener.

The localization problem may be simplified when the headphones havemicrophones directly on them (such as mounted on the hearing protectorear cup), so the listener can hear the environment instead of arecording or broadcast. Since the listener's head is already spatiallyfiltering the incoming sound, fewer parts of the HRTF are needed, whichwill require less processing. Left/right panning is stilldistinguishable. However, the use of headphones still alters thelistener's HRTF since normal pinna filtering is bypassed when amicrophone is placed on the outside of the hearing protection ear cup.

The localization problem can be further simplified by reducing therequired resolution. Research has focused on the ability to distinguishclosely spaced angles, e.g., 5° to 10°. Some sound localizationapplications require less angle resolution. Quadrant localizationattempts to resolve confusion between sources that are fore versus aft,in addition to the left versus right localization that is natural to thehead, as seen in FIG. 1.

Several efforts have been made to empirically measure individual HRTFs.The results of one such effort by AKG and IRCAM is available online. See“Listen HRTF Database.”http://recherche.ircam.fr/equipes/salles/listen/Institut De Recherche EtCoordination Acoustique/Musique, 2003. Last visited Oct. 23, 2013. Thisdatabase contains the head-related impulse response (HRIR) of aboutfifty test subjects. Analysis of this data confirms the fact that HRTFsare specific to each individual, but it also shows that there are somecommon factors among most HRTFs.

The data provided contains an HRIR for each test subject at 187combinations of azimuth and elevation angles. For the fore-aftlocalization problem only the HRIRs corresponding to sound sources inthe horizontal plane (i.e., elevation angle=0) and on the ipsilateralside (the ear closer to the sound source) are of interest. For each ear,there are 13 such HRIRs, equally spaced by 15°. Given a stationarylistener head, let 0° represent a point directly in front of the head.Similarly, 90° is directly left of the head, and 180° is directly behindthe head. For simplicity, it is assumed that the head is symmetric andrefer to all ears as if they were a left ear. In other words, 90° willalways represent a point on the ipsilateral side; no distinction wasmade between left and right ears.

Any two points in the horizontal plane that share the same ITD and IIDare referred to as “azimuth pairs.” Such pairs occur at angle pairs ofthe form θ_(fore)=90°−φ, θ_(aft)=90°+φ where φ is an arbitrary angleaway from the ear. There are six possible azimuth pairs available in thedatabase: 0°-180°, 15°-165°, 30°-150°, 45°-135°, 60°-120°, and 75°-105°.

For each of the 13 HRIRs of interest, an estimate of the spectraldensity using Welch's method was computed. See publication by Welch, P.D, entitled “The Use of Fast Fourier Transform for the Estimation ofPower Spectra: A Method Based on Time Averaging Over Short, ModifiedPeriodograms” IEEE Trans. Audio Electroacoustics, Vol. AU-15 (June1967), pp. 70-73. Subtracting the spectrum of the test stimulus from thepower spectral density of the HRIR shows the spectral shaping thatoccurs as a result of head geometry. It is assumed that, aftercompensation, the test stimulus had a flat frequency response, howeverthe gain of the stimulus signal is unknown. This information isimportant for determining which frequencies are attenuated or amplifiedby the system. Without a reference signal, only the relative power ofcertain frequencies versus other frequencies can be seen. In thisanalysis, the reference signal was chosen, somewhat arbitrarily, to be awhite signal at 30 dB.

FIGS. 2A-2F show plots of the average power spectral density over thelistener's ears for each of the six azimuth angles (e.g., 0 deg, 15 deg,30 deg, 45 deg, 60 deg and 75 deg) corresponding to sound sources infront of the listener. FIG. 3 shows a weighted average of the powerspectral density over the six forward azimuth angles. A weighting curvewas applied that emphasizes angles closer to 0°. FIGS. 4A-4F show plotsof the average power spectral density over the listener's ears for eachof the six azimuth angles (e.g., 180 deg, 165 deg, 150 deg, 135 deg, 120deg and 105 deg) corresponding to a sound sources behind the listener.FIG. 5 shows a weighted average of the power spectral density over thesix rear azimuth angles. A weighting curve was applied that emphasizesangles closer to 180°.

A goal of this research is quadrant localization in the horizontalplane. Duplex theory is helpful by determining the azimuth position of asound source. Whereas duplex theory takes advantage of binaural cues,aft-fore differentiation relies heavily on monaural cues. The spectralcues used to locate sounds can be detected by a single ear, and do notrequire simultaneous information from both ears. To eliminate the “coneof confusion,” the spectral differences between two points in the coneare evaluated, specifically, in the points where the cone of confusionintersects the horizontal plane. The information encoded in thesedifferences provide a key to quadrant localization.

FIGS. 6A-6F shows the spectral differences for each of the six azimuthpairs. Lines 82-87 represent the average spectral difference over allsubject ears. Lines 88, 89 represent one standard deviation above andbelow the average associated, and the individual points in each plotrepresent point-wise max and mins. As can be seen, the differences aremost pronounced where the angle difference increases. FIG. 7 shows thespectral differences averaged over all subject ears and all azimuthpairs on line 92, and lines 94, 95 represent one standard deviationabove and below respectively.

There is a need for an improved electronic hearing protector with soundlocalization.

SUMMARY OF THE INVENTION

An electronic hearing protector includes an ear cup assembly comprisinga front exterior microphone that provides a front microphone signal anda rear exterior microphone that provides a rear microphone signal. Aprocessor receives the front microphone signal and rear microphonesignal, and provides a front channel signal and a rear channel signal. Afirst filter, having a cut-off frequency value, receives the rearchannel signal and provides a first filtered signal. A second filter,having a notch frequency value, receives the first filtered signal andprovides a second filtered signal. A summer receives a first signalindicative of the front channel signal and a second signal indicative ofthe second filtered signal and provides a summed signal to a transducerthat provides an audio signal within the first ear cup indicative of thesummed signal that facilitates sound localization for the wearer of theelectronic hearing protector.

The first filter may comprise a first biquad filter configured andarranged as a low-pass filter, and the second filter may comprise asecond biquad filter configured and arranged as a notch filter.

The first biquad filter may have a cut-off frequency of about 12 kHz,and the second biquad filter may include a notch frequency located atabout 4.9 kHz.

The processor may also receive a second front microphone signal andsecond rear microphone signal, and provide a second front channel signaland a second rear channel signal. A low pass filter receives the secondrear channel signal and provides a second low pass filtered signal. Anotch filter receives the second low pass filtered signal and provides asecond notch filtered signal. A second summer receives a third signalindicative of the second front channel signal and a fourth signalindicative of the second notch filtered signal and provides a secondsummed signal to a second transducer that provides a second audio signalwithin the second ear cup indicative of the second summed signal.

The electronic hearing protector may also include front channelprocessing that includes a front channel shelf filter that receives thefirst front channel signal and provides a front channel filtered signal.A front channel notch filter receives the front channel filtered signaland provides the first signal.

The front channel shelf filter may comprise a biquad filter with acut-off frequency of about 1.9 kHz. The front channel notch filter maycomprise a second biquad filter with a notch of about 8.1 kHz.

In another embodiment, an electronic hearing protector includes an earcup assembly that comprises a front exterior microphone that provides afront microphone signal and a rear exterior microphone that provides arear microphone signal. A processor receives and digitizes the frontmicrophone signal and the rear microphone signal, and provides a frontchannel signal and a rear channel signal indicative thereof. A filterreceives and filters the rear channel signal, and has a cut-offfrequency value that provides a high-frequency roll-off and a notch at anotch filter frequency value that is less than the cut-off frequencyvalue, where the filter provides a filtered signal. A first signalindicative of the front channel signal and a second signal indicative ofthe filtered signal are summed, and a signal indicative of the resultantsum is provided to a transducer that provides an audio signal within thefirst ear cup indicative of the summed signal.

It is to be understood that the features mentioned above and those to beexplained below can be used not only in the respective combinationsindicated, but also in other combinations or in isolation.

These and other objects, features and advantages of the invention willbecome apparent in light of the detailed description of the embodimentthereof, as illustrated in the accompanying drawings.

DESCRIPTION OF THE DRAWINGS

The foregoing and other features of embodiments will become moreapparent from the following detailed description of embodiments whenread in conjunction with the accompanying drawings. In the drawings,like or identical reference numerals refer to like or identicalelements.

FIG. 1 is a pictorial illustration illustrating quadrants surroundingthe head of a listener;

FIGS. 2A-2F illustrate plots of the average power spectral density forsix different azimuth angles corresponding to sources in front of thelistener;

FIG. 3 illustrates a weighted average of the power spectral density oversix forward azimuth angles illustrated in FIGS. 2A-2F;

FIGS. 4A-4F illustrate plots of the average power spectral density overthe test listener's ears for six different azimuth angles correspondingto sources aft of the listener;

FIG. 5 illustrates a weighted average of the power spectral density overthe six aft azimuth angles illustrated in FIGS. 4A-4F;

FIGS. 6A-6F illustrate the spectral differences for each of the sixazimuth pairs set forth in FIGS. 2A-2F and FIG. 4A-4F;

FIG. 7 shows the spectral differences averaged over all subject ears andall azimuth pairs;

FIGS. 8A and 8B are perspective pictorial illustrations of an electronichearing protector with quadrant sound localization;

FIG. 9 is a block diagram illustration of signal processing associatedwith at least one of the ear cup assemblies of the electronic hearingprotector illustrated in FIGS. 8A-8B; and

FIG. 10 illustrates another embodiment of signal processing that mayassociated with at least one of the ear cup assemblies of the electronichearing protector illustrated in FIGS. 8A-8B.

DETAILED DESCRIPTION OF THE INVENTION

FIGS. 8A and 8B are perspective pictorial illustrations of an electronichearing protector/headset 100, which includes a first ear cup assembly102, a second ear cup assembly 104 and a head band 106 thatinterconnects the first and second ear cup assemblies 102, 104. Thefirst ear cup assembly 102 includes a first front microphone assembly107 and a first rear microphone assembly 108. The second ear cupassembly 104 includes a second front microphone assembly and a secondrear microphone assembly (not shown). Each of the microphone assembliesmay include an array microphone comprising a plurality ofomnidirectional microphones. Alternatively, directional microphones mayalso be used.

FIG. 9 is a block diagram illustration of signal processing 110associated with at least one of the ear cup assemblies 102, 104 (FIGS.8A and 8B). The front microphone assembly provides a front microphonesignal on a line 112 and the rear microphone assembly provides a rearmicrophone signal on a line 114. Array processor 116 processes the frontand rear microphone signals and provides a front channel signal on aline 118 and a rear channel signal on a line 120. In general, the arrayprocessor is configured and arranged as a delay and subtract beamformer.To provide the front channel signal on the line 118, the rear microphonesignal on the line 114 is delayed by a time equal to the propagationtime between the front and rear microphones (typically tens ofmicroseconds), then subtracted from the front microphone signal on theline 112. The resultant difference is provided as the front channelsignal on the line 118. To provide the rear channel signal on the line120, the front microphone signal on the line 112 is delayed by a timeequal to the propagation time between the front and rear microphone, andthe delayed front microphone signal is subtracted from the rearmicrophone signal on the line 114. The resultant difference is providedas the rear channel signal on the line 120.

In this embodiment a first filter 122 receives the rear channel signalon the line 120 and provides a first filtered signal on a line 124. Inone embodiment the first filter 122 may be configured as a low passfilter having a cut-off frequency value f_(ci). The first filter 122 maybe implemented as a biquad filter structure. However, it is contemplatedthat the first filter may be implemented in other digital filterembodiments, including for example a 1,024 tap FIR filter. One ofordinary skill will appreciate that the filter implementation for thefirst filter will primarily be based upon the processing power andmemory available. In addition, rather than or in addition to a FIRfilter, it is contemplated that alternative filter structures may alsobe used.

The first filtered signal on the line 124 is input to a second filter126, which provides a second filtered signal on a line 128. The secondfilter may be configured as a notch filter having a notch filter valuef_(n1). In one embodiment the second filter may also be implemented as abiquad. However, as set forth above with respect to the first filter, itis contemplated that the second filter 126 may be implemented a numberof different ways based upon the desired signal processing includingfiltering, and the processing power and memory available to thedesigner. It is further contemplated that the processing of the firstand second filters may be combined into a single filter that provides anoutput similar to the serially connected first and second filtersillustrated in FIG. 9.

The front channel signal on a line 118 and the second filtered signal ona line 128 may be summed by a summer 130, and a signal indicative of thesummed signal is output to a speaker 132 located within the ear cup. Theoutput by the speaker 132 provides enhanced audio with sufficient cuesto provide the headset wearer with sound localization (e.g., quadrantsound localization).

In one embodiment the first filter 122 may be configured as a low passfilter with a cut-off frequency located at about 12 kHz. The secondfilter 126 may be configured as a notch filter with a notch at about 4.9kHz. This embodiment provides spectral cues that allow the headsetwearer to determine front-to-back localization of sounds. The firstfilter H1 and the second filter H2 may be implemented as biquad filtersas follows:

${H\; 1(z)} = \frac{0.3 + {0.59\; z^{- 1}} + {0.3\; z^{- 2}}}{1 + {0.17\; z^{- 2}}}$${H\; 2(z)} = \frac{0.86 - {1.37\; z^{- 1}} + {0.86\; z^{- 2}}}{1 - {1.37\; z^{- 1}} + {0.71\; z^{- 2}}}$

In another embodiment, the ear cups 102, 104 (FIGS. 8A-8B) each includefront and rear microphones, and the headset provides the associatedsignal processing illustrated in FIG. 9 to the microphone signals fromeach ear cup to provide enhanced localization.

It is also contemplated that additional processing may be locatedbetween the summer 130 and speaker 132. For example, the additionalprocessing may include noise reduction, dynamics processing, equalizing,mixing and/or volume control, et cetera.

FIG. 10 illustrates another embodiment of signal processing 133 that maybe associated with at least one of the ear cup assemblies 102, 104 (FIG.10). This embodiment is substantially the same as the embodiment in FIG.9, with the principal exception that front channel signal path 134 alsoincludes processing to enhance localization and restore some of thenaturalness of human hearing. In this embodiment in the front channelsignal path includes a first front channel filter H3 136 that receivesthe front channel signal on the line 118 and provides a first frontchannel filtered signal on line 138 to a second front channel filter H4140. In this embodiment summer 142 sums the signal on the line 128 andthe output from the second front channel filter 140.

In an embodiment, the first front channel filter H3 136 may beconfigured as a high-shelf filter and the second front channel filter H4140 may be configured as a notch filter. Since processing power andmemory will be a premium, the first and second front channel filters136, 140 may be implemented as biquads. Of course, the implementation ofthe filter will be left to the filter designer based upon the desiredsystem signal processing and the available processing and memoryavailable to implement the filters.

In one embodiment, the first front channel filter H3 136 may be ahigh-shelf filter with a center frequency fc=1.9 kHz, and the secondfilter H4 140 may be a notch filter with a notch frequency fn=8.1 kHz,and both may be implemented as biquad filters as follows:

${H\; 3(z)} = \frac{1.39 + {2.59\; z^{- 1}} + {1.256\; z^{- 2}}}{1 - {1.826\; z^{- 1}} + {0.884\; z^{- 2}}}$${H\; 4(z)} = \frac{0.72 - {0.7\; z^{- 1}} + {0.72\; z^{- 2}}}{1 - {0.7\; z^{- 1}} + {0.43\; z^{- 2}}}$

In order to simulate a head-related transfer function, filters areimplemented that mimic the effects of the head and torso on the sound.These filters are applied to the outputs of a microphone array, wherebeamforming is used to yield forward facing inputs and rearward facinginputs. Because of the limited computing power available in a headsetapplication, the filters may be implemented as digital biquadraticfilters. This provides a balance between computational complexity andcustomizability.

Although discussed in the context of time domain processing, it iscontemplated that the filtering may also be performed in the frequencydomain. In addition, one of ordinary skill in the art will certainlyappreciate that higher order filters may be used. Rather than seriallyconnected filters as illustrated in FIGS. 9 and 10, each channel may usea single filter structure. For example, rather than using seriallyconnected first and second filters 122, 126 illustrated in FIG. 10, asingle filter may be used having a notch at about 4.9 kHz and a highfrequency roll off at about 12 kHz. The single filter may operate in thetime or frequency domains, and may be configured for example as a FIR orIIR filter. The notch may have quality factor Q of about 2.0.

The signal processing 110 and 133 of FIGS. 9 and 10 respectively, may beperformed by a microprocessor and complementary memory that storesexecutable programs that are executed in the microprocessor. One ofordinary skill will appreciate that rather than a microprocessor or aDSP, the signal processing may be performed by an ASIC or a gate array.The memory may include a main memory, a static memory, or a dynamicmemory. The memory may include, but is not limited to computer readablestorage media such as various types of volatile and non-volatile storagemedia, including but not limited to random access memory, read-onlymemory, programmable read-only memory, electrically programmableread-only memory, electrically erasable read-only memory, flash memory,magnetic tape or disk, optical media and the like. In one example, thememory includes a cache or random access memory for the processor. Inalternative examples, the memory is separate from the processor, such asa cache memory of a processor, the system memory, or other memory. Thememory may include or be an external storage device or database forstoring data. The memory is operable to store instructions executable bythe processor. The functions, acts or tasks illustrated in the figuresor described may be performed by the programmed processor executinginstructions stored in the memory. The functions, acts or tasks areindependent of the particular type of instructions set, storage media,processor or processing strategy and may be performed by software,hardware, integrated circuits, firmware, micro-code and the like,operating alone or in combination. Likewise, processing strategies mayinclude multiprocessing, multitasking, parallel processing and the like.While a preferred embodiment of the filtering is digital, it iscontemplated that analog filtering may also be used.

The electronic hearing protector 100 (FIGS. 8A and 8B) may be used forapplications such as shooting ranges, hunting, construction, heavyequipment operators and manufacturing. With respect to application suchas a shooting range, the electronic hearing protector headsets protectsthe wearer from impulse sound such as a gunshots, while allowing thewearer to hear and locate lower-level sound on the range, such as forexample instructions from the range officer. In addition, to the signalprocessing illustrated in FIGS. 9 and 10 to provide sound localization,it is contemplated that the electronic hearing protector may alsoinclude additional signal processing to attenuate impulse sound. Inaddition, each ear cup may include passive impulse noise attenuatingfeatures to further protect the hearing of the wearer from damagingimpulse noise. It is also contemplated that each ear cup may include aseparate volume control so the wearer can independently adjust thespeaker volume in each ear cup.

Although the invention has been illustrated and described with respectto several preferred embodiments thereof, various changes, omissions andadditions to the form and detail thereof, may be made therein, withoutdeparting from the spirit and scope of the invention.

What is claimed is:
 1. An electronic hearing protector, comprising: anear cup assembly comprising a front exterior microphone that provides afront microphone signal and a rear exterior microphone that provides arear microphone signal; a processor that receives the front microphonesignal and rear microphone signal, and provides a front channel signaland a rear channel signal; a first filter that receives the rear channelsignal, has a cut-off frequency value, and provides a first filteredsignal; a second filter that receives the first filtered signal, has anotch filter value, and provides a second filtered signal; and a summerthat receives a first signal indicative of the front channel signal anda second signal indicative of the second filtered signal and provides asummed signal to a transducer that provides an audio signal within thefirst ear cup indicative of the summed signal.
 2. The electronic hearingprotector of claim 1, where the first filter comprises a first biquadfilter configured and arranged as a low-pass filter.
 3. The electronichearing protector of claim 2, where the second filter comprises a secondbiquad filter configured and arranged as a notch filter.
 4. Anelectronic hearing protector, comprising: a first ear cup assemblycomprising a first front exterior microphone that provides a first frontmicrophone signal and a first rear exterior microphone that provides afirst rear microphone signal; a second ear cup assembly that includes asecond front exterior microphone that provides a second front microphonesignal and a second rear exterior microphone that provides a second rearmicrophone signal; a processor that receives the first front microphonesignal and first rear microphone signal, and provides a first frontchannel signal and a first rear channel signal; a first filter thatreceives the first rear channel signal and provides a low pass filteredsignal; a second filter that receives the low pass filtered signal andprovides a notch filtered signal; and a summer that receives a firstsignal indicative of the first front channel signal and a second signalindicative of the notch filtered signal and provides a summed signal toa transducer that provides an audio signal within the first ear cupindicative of the summed signal.
 5. The electronic hearing protector ofclaim 4, where the first filter comprises a first biquad filter with acut-off frequency of about 12 kHz.
 6. The electronic hearing protectorof claim 5, where the second filter comprises a second biquad filterwith a notch frequency of about 4.9 kHz.
 7. The electronic hearingprotector of claim 6, wherein the processor receives a second frontmicrophone signal and second rear microphone signal, and provides asecond front channel signal and a second rear channel signal associatedwith a second ear cup of the electronic hearing protector, and theelectronic hearing protector further comprises a low pass filter thatreceives the second rear channel signal and provides a second low passfiltered signal; a notch filter that receives the second low passfiltered signal and provides a second notch filtered signal; and asecond summer that receives a third signal indicative of the secondfront channel signal and a fourth signal indicative of the second notchfiltered signal and provides a second summed signal to a secondtransducer that provides a second audio signal within the second ear cupindicative of the second summed signal.
 8. The electronic hearingprotector of claim 4, further comprising: a front channel shelf filterthat receives the first front channel signal and provides a frontchannel filtered signal; and a front channel notch filter that receivesthe front channel filtered signal and provides the first signal.
 9. Theelectronic hearing protector of claim 8, wherein the front channel shelffilter comprises a biquad filter with a cut-off frequency of about 1.9kHz.
 10. The electronic hearing protector of claim 9, where the notchfilter comprises a second biquad filter with a notch of about 8.1 kHz.11. An electronic hearing protector, comprising: an ear cup assemblycomprising a front exterior microphone that provides a front microphonesignal and a rear exterior microphone that provides a rear microphonesignal; a processor that receives and digitizes the front microphonesignal and the rear microphone signal, and provides a front channelsignal and a rear channel signal; a filter that receives the rearchannel signal and has a cut-off frequency value that provides ahigh-frequency roll-off and a notch at a notch filter frequency valuethat is less than the cut-off frequency value, where the filter providesa filtered signal; and a summer that receives a first signal indicativeof the front channel signal and a second signal indicative of thefiltered signal and provides summed signal to a transducer that providesan audio signal within the first ear cup indicative of the summedsignal.
 12. The electronic hearing protector of claim 11, wherein theprocessor receives a second front microphone signal and second rearmicrophone signal, and provides a second front channel signal and asecond rear channel signal associated with a second ear cup of theelectronic hearing protector, and the electronic hearing protectorfurther comprises a second filter that receives the second rear channelsignal and has a second cut-off frequency value that provides ahigh-frequency roll-off and a notch at a second notch filter frequencyvalue that is less than the cut-off frequency value, where the secondfilter provides a second filtered signal; and a second summer thatreceives a third signal indicative of the second front channel signaland a fourth signal indicative of the second filtered signal andprovides a second summed signal to a second transducer that provides asecond audio signal within the second ear cup indicative of the secondsummed signal.
 13. The electronic hearing protector of claim 12, furthercomprising: a front channel shelf filter that receives the first frontchannel signal and provides a front channel filtered signal; and a frontchannel notch filter that receives the front channel filtered signal andprovides the first signal.
 14. The electronic hearing protector of claim13, where the first cut-off frequency value is about 12 kHz, and thenotch filter frequency value is about 4.9 kHz.
 15. The electronichearing protector of claim 14, wherein the front channel shelf filtercomprises a cut-off frequency value of about 1.9 kHz, and notch filterfrequency value of about 8.1 kHz.